The present invention relates to a photo detector and particularly relates to a photo detector for optical chip to chip communication.
The design of on-chip optical inputs and outputs for optical chip to chip communication is restricted by the fact that the implementation of photo detectors in CMOS integrated circuits is very challenging. Silicon is a suitable semiconductor for short wavelength photo detectors. A CMOS process, or even better a BICMOS process, offers a range of different designs of pn-junctions which can be used as photo detectors. However, the doping levels for these pn-junctions are very high. As a result, a depletion region of these pn-junctions is very narrow, for example approximately 50 nm to 100 nm, and, as a consequence, the volume which can be used to absorb photons is small. This results in a small responsivity of the photo detector. The narrow depletion region further results in high junction capacitances, which severely limit a frequency response of such a photo detector.
When implementing photo detectors in silicon on insulator, or abbreviated SOI, the CMOS manufacturing process is even more challenging. In a SOI CMOS manufacturing process the bulk silicon only serves as a mechanical substrate and has no electrical function. The active silicon layer may have a thickness of only, for example, approximately 70 nm. Silicon has an absorption length of roughly 15 μm for light with a wavelength of 850 nm. As a result, 1/e of the light is absorbed after 15 μm in silicon. Thus, a 70 nm silicon layer is virtually transparent for light of that wavelength. A shorter wavelength results in a shorter absorption length. However, the 70 nm silicon layer is too thin for all practical wavelengths.
It is therefore a challenge to provide a photo detector that is simple and efficient.